claim exp should be integer (#13582)

claim exp can be 

- float64
- json.Number

As per OIDC spec https://openid.net/specs/openid-connect-core-1_0.html#IDToken

Avoid using strings since the upstream library only supports these two types now.

internal/config/identity/openid/jwt.go

@@ -287,8 +287,7 @@ func updateClaimsExpiry(dsecs string, claims map[string]interface{}) error {
 		defaultExpiryDuration = time.Unix(expAt, 0).UTC().Sub(time.Now().UTC())
 	} // else honor the specified expiry duration.
 
-	expiry := time.Now().UTC().Add(defaultExpiryDuration).Unix()
-	claims["exp"] = strconv.FormatInt(expiry, 10) // update with new expiry.
+	claims["exp"] = time.Now().UTC().Add(defaultExpiryDuration).Unix() // update with new expiry.
 	return nil
 }
 

internal/config/identity/openid/jwt_test.go

@@ -19,12 +19,15 @@ package openid
 
 import (
 	"crypto"
+	"encoding/base64"
 	"encoding/json"
 	"net/url"
 	"sync"
 	"testing"
 	"time"
 
+	jwtg "github.com/golang-jwt/jwt"
+	jwtm "github.com/minio/minio/internal/jwt"
 	xnet "github.com/minio/pkg/net"
 )
 
@@ -202,3 +205,28 @@ func TestDefaultExpiryDuration(t *testing.T) {
 		}
 	}
 }
+
+func TestExpCorrect(t *testing.T) {
+	signKey, _ := base64.StdEncoding.DecodeString("NTNv7j0TuYARvmNMmWXo6fKvM4o6nv/aUi9ryX38ZH+L1bkrnD1ObOQ8JAUmHCBq7Iy7otZcyAagBLHVKvvYaIpmMuxmARQ97jUVG16Jkpkp1wXOPsrF9zwew6TpczyHkHgX5EuLg2MeBuiT/qJACs1J0apruOOJCg/gOtkjB4c=")
+
+	claimsMap := jwtm.NewMapClaims()
+	claimsMap.SetExpiry(time.Now().Add(time.Minute))
+	claimsMap.SetAccessKey("test-access")
+	if err := updateClaimsExpiry("3600", claimsMap.MapClaims); err != nil {
+		t.Error(err)
+	}
+	// Build simple toke with updated expiration claim
+	token := jwtg.NewWithClaims(jwtg.SigningMethodHS256, claimsMap)
+	tokenString, err := token.SignedString(signKey)
+	if err != nil {
+		t.Error(err)
+	}
+
+	// Parse token to be sure it is valid
+	err = jwtm.ParseWithClaims(tokenString, claimsMap, func(*jwtm.MapClaims) ([]byte, error) {
+		return signKey, nil
+	})
+	if err != nil {
+		t.Error(err)
+	}
+}